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</div><div id='content' class='page-1'><div class='row'><div class='row-fluid'><div class='col-lg-3'><ul class='nav-sidebar'><li><a href='./'>Blog index</a></li><li class='active'><a href='#doc'>GPars meets Virtual Threads</a></li><li><a href='#_parallel_collections' class='anchor-link'>Parallel Collections</a></li><li><a href='#_agents' class='anchor-link'>Agents</a></li><li><a href='#_actors' class='anchor-link'>Actors</a></li><li><a href='#_dataflow' class='anchor-link'>Dataflow</a></li><li><a href='#_conclusion' class='anchor-link'>Conclusion</a></li></ul><br/><ul class='nav-sidebar'><li style='padding: 0.35em 0.625em; background-color: #eee'><span>Related posts</span></li><li><a href='./groovy-list-processing-cheat-sheet'>Groovy List Processing Cheat Sheet</a></li><li><a href='./fruity-eclipse-collections'>Fruity Eclipse Collections</a></li></ul></div><div class='col-lg-8 col-lg-pull-0'><a name='doc'></a><h1>GPars meets Virtual Threads</h1><p><span>Author: <i>Paul King</i></span><br/><span>Published: 2022-06-15 11:28AM (Last updated: 2023-04-14 06:23PM)</span></p><hr/><div id="preamble">
<div class="sectionbody">
<div class="paragraph">
<p><span class="image right"><img src="img/gpars_logo.png" alt="gpars" width="150"></span>
An exciting feature coming in JDK21 is Virtual Threads
(<a href="https://openjdk.java.net/jeps/444">JEP 444</a>).
It has been previewed in JDK19 (<a href="https://openjdk.java.net/jeps/425">JEP 425</a>)
and JDK20 (<a href="https://openjdk.java.net/jeps/436">JEP 436</a>) but JDK21 will be the first version
that the feature is available without using Java&#8217;s preview switch.
The examples in this blog were tested with Groovy 4.0.11 using JDK version 21-ea+18-1480
(and using JDK19 with preview features enabled).</p>
</div>
<div class="paragraph">
<p>Virtual threads work well with my favourite Groovy parallel
and concurrency library <a href="http://gpars.org/">GPars</a>. GPars has been
around a while (since Java 5 and Groovy 1.8 days) but still has
many useful features. Let&#8217;s have a look at a few examples.</p>
</div>
<div class="paragraph">
<p>If you want to try these out, use the latest JDK21ea version which has virtual
thread support as standard. Or use a recent JDK19-20 version
and enable <em>preview</em> features with your Groovy tooling.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_parallel_collections">Parallel Collections</h2>
<div class="sectionbody">
<div class="paragraph">
<p>First a refresher, we&#8217;ll first look at using the GPars parallel collections feature
with normal threads. Let&#8217;s start with a list of numbers:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">var nums = [1, 2, 3]</code></pre>
</div>
</div>
<div class="paragraph">
<p>To calculate the list of squares of our original numbers in
parallel with normal threads, we use the <code>GParsPool.withPool</code> method as follows:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">withPool {
assert nums.collectParallel{ it ** 2 } == [1, 4, 9]
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>For any Java readers, don&#8217;t get confused with the <code>collectParallel</code>
method name. Groovy&#8217;s <code>collect</code> method (naming inspired by
Smalltalk) is the equivalent of Java&#8217;s <code>map</code> method. So, the
equivalent Groovy code using the Java streams API would be
something like:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">assert nums.parallelStream().map(n -&gt; n ** 2).toList() == [1, 4, 9]</code></pre>
</div>
</div>
<div class="paragraph">
<p>Now, let&#8217;s bring virtual threads into the picture. Luckily,
GPars parallel collection facilities provide a hook for using
an <em>existing</em> custom executor service. This makes using virtual
threads for such code easy. First we create our pool (backed by virtual threads):</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">var vtPool = Executors.newVirtualThreadPerTaskExecutor()</code></pre>
</div>
</div>
<div class="paragraph">
<p>Now, we can use it as follows:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">withExistingPool(vtPool) {
assert nums.collectParallel{ it ** 2 } == [1, 4, 9]
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>Or we can use one of the other many <em>'*Parallel'</em> methods, in this case <code>findAllParallel</code>:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">var isEven = n -&gt; n % 2 == 0
withExistingPool(vtPool) {
assert (1..9).findAllParallel(isEven) == (2..8).step(2)
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>Nice! Using virtual threads is very simple!</p>
</div>
<div class="paragraph">
<p>Let&#8217;s look at one more example, the <a href="https://en.wikipedia.org/wiki/Fizz_buzz">FizzBuzz</a> example:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">var result = withExistingPool(vtPool) {
(1..15).collectParallel {
switch(it) {
case { it % 15 == 0 } -&gt; 'FizzBuzz'
case { it % 5 == 0 } -&gt; 'Buzz'
case { it % 3 == 0 } -&gt; 'Fizz'
default -&gt; it
}
}.join(',')
}
assert result == '1,2,Fizz,4,Buzz,Fizz,7,8,Fizz,Buzz,11,Fizz,13,14,FizzBuzz'</code></pre>
</div>
</div>
<div class="paragraph">
<p>Now, let&#8217;s move onto some examples which might be
less familiar to Java developers.</p>
</div>
<div class="paragraph">
<p>GPars has additional features for providing custom thread pools
and the remaining examples rely on those features. The current
version of GPars doesn&#8217;t have a <code>DefaultPool</code> constructor that
takes a vanilla executor service; so, we&#8217;ll write our own class:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">@AutoImplement
class VirtualPool implements Pool {
private final ExecutorService pool = Executors.newVirtualThreadPerTaskExecutor()
int getPoolSize() { pool.poolSize }
void execute(Runnable task) { pool.execute(task) }
ExecutorService getExecutorService() { pool }
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>It is essentially a delegate from the GPars <code>Pool</code> interface
to the virtual threads executor service.</p>
</div>
<div class="paragraph">
<p>We&#8217;ll use this in the remaining examples.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_agents">Agents</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Agents provide a thread-safe non-blocking wrapper around an
otherwise potentially mutable shared state object. They are
inspired by agents in Clojure.</p>
</div>
<div class="paragraph">
<p>In our case we&#8217;ll use an agent to "protect" a plain <code>ArrayList</code>.
For this simple case, we could have used some synchronized list,
but in general, agents eliminate the need to find thread-safe
implementation classes or indeed care at all about the thread
safety of the underlying wrapped object.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">var mutableState = [] // a non-synchronized mutable list
var agent = new Agent(mutableState)
agent.attachToThreadPool(new VirtualPool()) // omit line for normal threads
agent { it &lt;&lt; 'Dave' } // one thread updates list
agent { it &lt;&lt; 'Joe' } // another thread also updating
assert agent.val.size() == 2</code></pre>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_actors">Actors</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Actors allow for a message passing-based concurrency model.
The actor model ensures that at most one thread processes
the actor&#8217;s body at any time. The GPars API and DSLs for actors
are quite rich supporting many features. We&#8217;ll look at a simple
example here.</p>
</div>
<div class="paragraph">
<p>GPars manages actor thread pools in groups.
Let&#8217;s create one backed by virtual threads:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">var vgroup = new DefaultPGroup(new VirtualPool())</code></pre>
</div>
</div>
<div class="paragraph">
<p>Now we can write an encrypting and decrypting actor pair as follows:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">var decryptor = vgroup.actor {
loop {
react { String message -&gt;
reply message.reverse()
}
}
}
var console = vgroup.actor {
decryptor &lt;&lt; 'lellarap si yvoorG'
react {
println 'Decrypted message: ' + it
}
}
console.join() // output: Decrypted message: Groovy is parallel</code></pre>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_dataflow">Dataflow</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Dataflow offers an inherently safe and robust declarative
concurrency model. Dataflows are also managed via thread
groups, so we&#8217;ll use <code>vgroup</code> which we created earlier.</p>
</div>
<div class="paragraph">
<p>For the sake of an example, we&#8217;ll create a scenario where two
tasks are producing some results and a third task is adding the results
of the other tasks.</p>
</div>
<div class="paragraph">
<p><span class="image"><img src="img/gpars_dataflow.png" alt="gpars dataflow"></span></p>
</div>
<div class="paragraph">
<p>We have three logical tasks which can run in parallel and perform
their work. The tasks need to exchange data and they do so using
<em>dataflow variables</em>. Think of dataflow variables as one-shot
channels safely and reliably transferring data from producers to
their consumers.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">var df = new Dataflows()
vgroup.with {
task {
df.z = df.x + df.y
}
task {
df.x = 10
}
task {
df.y = 5
}
assert df.z == 15
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>This code is declarative in style. We can specify the three tasks in any order.
We aren&#8217;t giving any indication of which tasks should occur first.
The dataflow framework works out how to schedule the individual
tasks and ensures that a task&#8217;s input variables are ready when
needed.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_conclusion">Conclusion</h2>
<div class="sectionbody">
<div class="paragraph">
<p>We have had a quick glimpse at using virtual threads with Groovy
and GPars. It is still early days with virtual threads, so expect
much more to emerge as JDK21 becomes more mainstream.</p>
</div>
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